It is sought more particularly here below in this document to describe problems existing in the field of marine seismic exploration. The invention of course is not limited to this particular field of application but is of interest for any technique that has to cope with closely related or similar issues and problems.
The operations of acquiring seismic data on site conventionally use networks of sensors (also designated as “hydrophones”) distributed along cables in order to form linear acoustic antennas (also called “seismic streamers”) towed by a seismic vessel. To collect geophysical data in a marine environment, submerged air guns (also called seismic sources), or more generally acoustic transmitters meant to generate a radiated acoustic pressure pulse under water, are used to gather geophysical information relating to the substrata located offshore. During marine seismic surveys, several air guns are towed behind a marine vessel. The shock wave generated by the air guns propagates into the ground where they are refracted and reflected back to the top. Antennas composed by sensors are used to log the returning wave and to convert and transmit these signals. When processed, this data will help to characterize the geophysical structure of the substrata. For a deep-water survey, several guns are deployed underneath a floatation device, within an arrangement previously calculated and simulated in order to build an overall expected acoustic pressure pulse. All air guns are activated simultaneously or not.
Generation of an acoustic signal in water by the airgun is based on a compressed gas release mechanism explained below with reference to FIG. 1.
An airgun 10 typically comprises a pneumatic chamber (also hereafter called “firing chamber”) 12 intended to contain a compressed gas volume that can be released to the water through exhaust ports, for example two pneumatic exhaust ports 14a and 14b communicating with the surrounding water. The pneumatic exhaust ports 14a and 14b are holes through which the gas volume (also designated as “pneumatic volume” or “firing volume”) is released from the firing chamber 12 into the surrounding water to create a bubble. Arrows 15 represent the gas volume thus released from the firing chamber 12. The bubble creates an acoustic pressure wave, also hereafter called acoustic signal. To that end, the airgun 10 comprises a movable shuttle 16 which can be moved between two extreme positions along its translational axis X, namely:                a closed position (FIG. 1A) in which the compressed gas volume is enclosed within the firing chamber 12,        an open position (FIG. 1C) in which the compressed gas volume is released out of the firing chamber 12 through the pneumatic exhaust ports 14a and 14b, to generate the bubble, which then creates the acoustic signal in the surrounding water.        
FIG. 1B shows the airgun 10 in an intermediate configuration in which the movable shuttle 16 is a half-open position. The airgun 10 is being opening.
Usually, the airgun 10 further comprises a hydraulic chamber 18, located ahead of the shuttle 16 in closed position, containing a liquid volume that ensures the brake of the movable shuttle 16 during the opening phase. The hydraulic chamber 18 directly communicates with the exhaust ports 14a and 14b. 
The phase during which the shuttle 16 moves between the closed and open positions is commonly referred to as “opening phase” or “firing phase” of the airgun. During this opening phase, the shuttle 16 acquires a high velocity before uncovering the exhaust ports 14a and 14b. High compressed gas volume 15 is then released into the surrounding water to create a bubble that generates the acoustic signal. In parallel, a part of the liquid volume from the hydraulic chamber 18 is also released through the exhaust ports 14a and 14b (represented by arrows referenced 13). The shuttle opening mechanism is triggered by actuating a solenoid valve (referenced 11 in FIGS. 2A and 2B).
Once the firing phase completed, the firing chamber 12 being no longer under pressure, the shuttle 16 returns into its closed position to seal the firing chamber 12. The firing chamber 12 is then filled up to the required pressure with compressed gas by means of a return chamber (referenced 19 in FIGS. 2A and 2B), before launching again the opening phase of the shuttle 16.
At the rear of the airgun, it is common to find embedded electronics and various sensors.
A well-known problem of the prior art airguns is the control of output acoustic signals. Indeed, it is important to be able to control accurately shape of acoustic signal generated by the airgun as function of expected needs.
The patent document U.S. Pat. No. 7,321,527 proposes an airgun whose output acoustic signal is controlled by means of an adjustment of the pneumatic structural features, with the aim of reducing high frequency range of acoustic signals. High frequency signals are generally considered unwanted signals (i.e. noise) as they are outside of the frequency range usually used in marine seismic exploration. In addition, they generate underwater noise pollution that they may disrupt the marine wildlife. In order to meet this need, it is proposed in that document to configure The pneumatic chamber and/or pneumatic exhaust ports to adjust the gas rate released into water during the opening phase of the shuttle, so as to create a pneumatic exhaust area at a non-linear rate. The amplitude of the unwanted seismic frequencies emitted in water can be then reduced by adjusting the slope of the radiated acoustic pressure,
A drawback of this known solution is that the range of modulation of acoustic signal is relatively limited. It further requires an accurate adjustment of the pneumatic structural features of the airgun, especially since the pneumatic forces that participate to the acoustic signal creation are not easily controllable.
In addition, this known technique provides a static solution and the output acoustic signal cannot be remotely tuned, for example from a control unit placed on the seismic vessel, which is not optimal.